Technical Abstract:
In this research, four chemicals, urea, ammonium sulfate, dicyandiamide, and melamine, were mixed into liquid nonfat milk at concentrations starting from 0.1% to a maximum concentration determined for each chemical according to its maximum solubility, and two Raman spectrometers—a commercial Nicolet Raman system and an in-house Raman Chemical Imaging (RCI) system—were used to acquire Raman shift spectra for these mixture samples. These chemicals are potential adulterants that could be used to artificially elevate protein measurements of milk products evaluated by the Kjeldahl method. Baseline subtraction was employed to eliminate milk intensity, and the normalized Raman intensity was calculated from the specific Raman shift from the spectrum of solid chemical. Linear relationships were found to exist between the normalized Raman intensity and chemical concentrations. The linear regression coefficients (R2) ranged from 0.9111 to 0.998. Although slightly higher R2 values were calculated for regressions using spectral intensities measured by the Nicolet system compared to those using measurements from the RCI system, the results from the two systems were similar and comparable. A very low concentration of melamine (400 ppm) in milk was also found to be detectable by both systems. Raman sensitivity of Nicolet Raman system was estimated from normalized Raman intensity and slope of regression line in this study. Chemicals (0.2%) were dissolved in milk and detected the normalized Raman intensity. Melamine was found to have the highest Raman sensitivity, with the highest values for normalized Raman intensity (0.09) and regression line slope (57.04).